Various methods for reducing the amount of carbon monoxide in the mainstream smoke of a cigarette during smoking have been proposed. For example, British Patent No. 863,287 describes methods for treating tobacco prior to the manufacture of tobacco articles, such that incomplete combustion products are removed or modified during smoking of the tobacco article. In addition, cigarettes comprising absorbents, generally in a filter tip, have been suggested for physically absorbing some of the carbon monoxide. Cigarette filters and filtering materials are described, for example, in U.S. Reissue Pat. No. RE 31,700; U.S. Pat. No. 4,193,412; British Patent No. 973,854; British Patent No. 685,822; British Patent No. 1,104,993 and Swiss patent 609,217. However, such methods are usually not completely efficient.
Catalysts for the conversion of carbon monoxide to carbon dioxide are described, for example, in U.S. Pat. Nos. 4,317,460, 4,956,330; 5,258,330; 4,956,330; 5,050,621; and 5,258,340, as well as in British Patent No. 1,315,374. The disadvantages of incorporating a conventional catalyst into a cigarette include the large quantities of oxidant that need to be incorporated into the filter to achieve considerable reduction of carbon monoxide. Moreover, if the ineffectiveness of the heterogeneous reaction is taken into account, the amount of the oxidant required would be even larger.
Metal oxides, such as iron oxide have also been incorporated into cigarettes for various purposes. See, for example, International Publications WO 87/06104 and WO 00/40104, as well as U.S. Pat. Nos. 3,807,416 and 3,720,214. Iron oxide has also been proposed for incorporation into tobacco articles, for a variety of other purposes. For example, iron oxide has been described as particulate inorganic filler (e.g. U.S. Pat. Nos. 4,197,861; 4,195,645; and 3,931,824), as a coloring agent (e.g. U.S. Pat. No. 4,119,104) and in powder form as a burn regulator (e.g. U.S. Pat. No. 4,109,663). In addition, several patents describe treating filler materials with powdered iron oxide to improve taste, color and/or appearance (e.g. U.S. Pat. Nos. 6,095,152; 5,598,868; 5,129,408; 5,105,836 and 5,101,839). However, the prior attempts to make cigarettes incorporating metal oxides, such as FeO or Fe2O3 have not led to the effective reduction of carbon monoxide in mainstream smoke.
Despite the developments to date, there remains a need for improved and more efficient methods and compositions for reducing the amount of carbon monoxide in the mainstream smoke of a cigarette during smoking. Preferably, such methods and composition should not involve expensive or time consuming manufacturing and/or processing steps. More preferably, it should be possible to catalyze or oxidize carbon monoxide not only in the filter region of the cigarette, but also along the entire length of the cigarette during smoking.